Insulating paperboard

ABSTRACT

An insulating paperboard contains at least one layer of cellulose fibers. The one layer is at least partially composed of crosslinked fibers and processed cellulosic fibers. The paperboard provides sufficient insulation to provide a ΔT across the paperboard of at least 3.6° C. at 0.5 mm. A hot cup may be produced from the insulating paperboard.

FIELD

The present application relates to an insulating paperboard, and more particularly to an insulating paperboard containing crosslinked cellulosic fibers and processed cellulosic fibers.

BACKGROUND

Hot foods, particularly hot liquids, are commonly served and consumed in disposable containers. These containers are made from a variety of materials including paperboard and foamed polymeric sheet material. One of the least expensive sources of paperboard material is cellulose fibers. Cellulose fibers are employed to produce excellent paperboards for the production of hot cups, noodle cups, press-molded paperboard plates and bowls, and other food and beverage containers. Conventional paperboard produced from cellulosic fibers, however, is relatively dense, and therefore, transmits heat more readily than, for example, foamed polymeric material. Thus, hot liquids are often served in doubled cups of conventional paperboard or in cups with sleeves.

It is desirable to possess an insulating paperboard produced from cellulosic material that has good insulating characteristics, that will allow the user to sense that food in the container is warm or hot and at the same time will allow the consumer of the food or beverage in the container to hold the container for a lengthy period of time without the sensation of excessive temperature. It is further desirable to provide an insulating paperboard that can be tailored to provide a variety of insulating characteristics so that the temperature drop across the paperboard can be adjusted for a particular end use.

BRIEF DESCRIPTION OF THE DRAWINGS

This application will become more readily appreciated and understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-sectional view of a two-ply paperboard which can be constructed in accordance with the present application;

FIG. 2 is an isometric view of a hot cup made from the paperboard similar to that shown in FIG. 1 with a portion cut away; and

FIG. 3 is an enlarged cross-sectional view of a portion of the paperboard used to make the hot cup shown in FIG. 2.

DETAILED DESCRIPTION

Referring to FIG. 1, the substrate 10 for the insulating paperboard 12 of the present application is produced in a conventional manner from readily available fibers such as cellulosic fibers. The paperboard of the present application can be made in a single-ply, a two-ply construction, or a multi-ply construction, as desired.

The distinguishing characteristic of the present application is that at least one ply, 14, of the insulating paperboard, whether a single-ply or a multiple-ply structure, contains a mixture of crosslinked cellulosic fibers and processed cellulosic fibers in addition to chemical pulp fibers. The mixture of crosslinked cellulosic fibers and processed cellulosic fibers increase the insulating characteristics of the board. As defined herein chemical pulp fibers useable in the present application are derived primarily from wood pulp and may be refined. Other sources such as from kenaf and straw pulp may also be used. Suitable wood pulp fibers for use with the application can be obtained from well-known chemical processes such as the kraft and sulfite processes, with or without subsequent bleaching. Softwoods and hardwoods can be used. Details of the selection of wood pulp fibers are well known to those skilled in the art. For example, suitable cellulosic fibers (chemical pulp fibers) produced from southern pine that are useable in the present application are available from a number of companies including Weyerhaeuser Company under the designations C-Pine, Chinook, CF416, FR416, and NB416. A bleached Kraft wet lap pulp, KKT, Prince Albert Softwood and Grande Prairie Softwood, all manufactured by Weyerhaeuser are examples of northern softwoods that can be used. As used herein, processed cellulosic fibers include fibers that are 1) chemically processed to change the cellulose from Cellulose 1 to Cellulose 11, such as mercerized and mercerized flash dried fibers in which the mercerization is conducted as one stage in the bleaching process. Mercerized fibers such as HPZ and mercerized flash dried fibers such as HPZ III, both manufactured by Buckeye Technologies, Memphis Tenn., and Porosinier-J-HP available from Rayonier Performance Fibers Division, Jessup, Ga. are suitable for use in the present application. These mercerized softwood pulps have an α-cellulose purity of 95% or greater and are stiff fibers. Processed fibers also include 2) mechanically and chemimechanically treated fibers such as chemithermomechanical pulp fibers (CTMP), bleached chemithermomechanical pulp fibers (BCTMP), thermomechanical pulp fibers (TMP), refiner groundwood pulp fibers and groundwood pulp fibers. Examples of these pulps are TMP (thermomechanical pulp) made by Bowater, Greenville, S.C., a TMP (thermomechanical pulp) made by Weyerhaeuser, Federal Way, Wash., made by passing wood chips through three stages of dual refiners, and a CTMP (chemi-thermomechanical pulp) obtained from NORPAC, Longview, Wash., sold as a CTMP NORPAC Newsprint Grade; the brightness is from 53 to 75. Other processed fibers include jet dried cellulosic fibers and treated jet dried cellulosic fibers manufactured by the Weyerhaeuser Company by the method described in U.S. application Ser. No. 10/923,447 filed Aug. 20, 2004. In this method a slurry of pulp fibers is dewatered to a consistency of approximately 34% and then passed through a jet drier having an inlet temperature of approximately 190° C. to 400° C. an outlet temperature of 50° C. to 205° C. and a steam pressure of approximately 1082 kPa (157 psig). These fibers are twisted kinked and curled. Additional processed fibers include flash dried and treated flash dried fibers as described in U.S. Pat. No. 6,837,970, Mixtures of processed fibers with crosslinked cellulosic fibers can also be used.

The preferred bulky fibers for use in the invention are crosslinked cellulosic fibers. Any one of a number of crosslinking agents and crosslinking catalysts, if necessary, can be used to provide the crosslinked fibers to be included in the layer. The following is a representative list of useful crosslinking agents and catalysts.

Suitable crosslinking agents include carboxylic acid crosslinking agents such as polycarboxylic acids. Polycarboxylic acid crosslinking agents (e.g., citric acid, propane tricarboxylic acid, and butane tetracarboxylic acid) and catalysts are described in U.S. Pat. Nos. 3,526,048; 4,820,307; 4,936,865; 4,975,209; and 5,221,285. The use of C₂-C₈ polycarboxylic acids that contain at least three carboxyl groups (e.g., citric acid and oxydisuccinic acid) as crosslinking agents is described in U.S. Pat. Nos. 5,137,537; 5,183,707; 5,190,563; 5,562,740; and 5,873,979.

Polymeric polycarboxylic acids are also suitable crosslinking agents. Suitable polymeric polycarboxylic acid crosslinking agents are described in U.S. Pat. Nos. 4,391,878; 4,420,368; 4,431,481; 5,049,235; 5,160,789; 5,442,899; 5,698,074; 5,496,476; 5,496,477; 5,728,771; 5,705,475; and 5,981,739. Polyacrylic acid and related copolymers as crosslinking agents are described U.S. Pat. Nos. 5,549,791 and 5,998,511. Polymaleic acid crosslinking agents are described in U.S. Pat. No. 5,998,511 and U.S. Pat. No. 6,582,553.

Suitable specific crosslinking agents include polycarboxylic acid crosslinking agents such as citric acid, tartaric acid, malic acid, succinic acid, glutaric acid, citraconic acid, itaconic acid, tartrate monosuccinic acid, maleic acid, polyacrylic acid, polymethacrylic acid, polymaleic acid, polymethylvinylether-co-maleate copolymer, polymethylvinylether-co-itaconate copolymer, copolymers of acrylic acid, and copolymers of maleic acid. Other suitable crosslinking agents are described in U.S. Pat. Nos. 5,225,047; 5,366,591; 5,556,976; and 5,536,369.

Suitable crosslinking catalysts can include acidic salts, such as ammonium chloride, ammonium sulfate, aluminum chloride, magnesium chloride, magnesium nitrate, and alkali metal salts of phosphorous-containing acids. In one embodiment, the crosslinking catalyst is sodium hypophosphite.

The crosslinking agent is applied to the cellulosic fibers as they are being produced in an amount sufficient to effect intrafiber crosslinking. The amount applied to the cellulosic fibers may be from about 1% to about 25% by weight based on the total weight of fibers. In one embodiment, crosslinking agent in an amount from about 4% to about 6% by weight based on the total weight of fibers. Mixtures or blends of crosslinking agents and catalysts can also be used.

In addition to fibrous materials, the paperboard of the invention may optionally include a binding agent. Suitable binding agents are soluble in, dispersible in, or form a suspension in water. Suitable binding agents include those agents commonly used in the paper industry to impart wet and dry tensile and tearing strength to such products. Suitable wet strength agents include cationic modified starch having nitrogen-containing groups (e.g., amino groups), such as those available from National Starch and Chemical Corp., Bridgewater, N.J.; latex; wet strength resins, such as polyamide-epichlorohydrin resin (e.g., KYMENE 557LX, Hercules, Inc., Wilmington, Del.), and polyacrylamide resin (see, e.g., U.S. Pat. No. 3,556,932 and also the commercially available polyacrylamide marketed by American Cyanamid Co., Stanford, Conn., under the trade name PAREZ 631 NC); urea formaldehyde and melamine formaldehyde resins; and polyethylenimine resins. A general discussion on wet strength resins utilized in the paper field, and generally applicable in the present invention, can be found in TAPPI monograph series No. 29, “Wet Strength in Paper and Paperboard”, Technical Association of the Pulp and Paper Industry (New York, 1965).

Other suitable binding agents include starch, modified starch, polyvinyl alcohol, polyvinyl acetate, polyethylene/acrylic acid copolymer, acrylic acid polymers, polyacrylate, polyacrylamide, polyamine, guar gum, oxidized polyethylene, polyvinyl chloride, polyvinyl chloride/acrylic acid copolymers, acrylonitrile/butadiene/styrene copolymers, and polyacrylonitrile. Many of these will be formed into latex polymers for dispersion or suspension in water.

Paperboard of the present application may have a broad set of characteristics. For example, in one embodiment its basis weight can range from about 200 gsm to about 500 gsm, in another embodiment the basis weight ranges from 250 gsm to 400 gsm. In yet another embodiment the basis weight of the paperboard is equal to or greater than about 350 gsm. In one embodiment the insulating paperboard has a density of less than 0.5 g/cc, in another embodiment the density is from about 0.3 g/cc to about 0.45 g/cc, and in another embodiment the density is from 0.35 g/cc to 0.40 g/cc.

When at least one ply of the paperboard contains a mixture of crosslinked cellulosic fibers and processed cellulosic fibers in accordance with the present application, advantageous temperature drop characteristics can be achieved. These temperature drop characteristics can be achieved by altering the amount of these two fibers introduced into the paperboard, by adjusting the basis weight of the paperboard, by adjusting the caliper of the paperboard after it has been produced by running it, for example, through nip rolls, and of course, by varying the number and thickness of additional plies incorporated in the paperboard structure. In one embodiment the paperboard has a caliper greater than or equal to 0.5 mm, a basis weight equal to or greater than 200 gsm, and a density less than 0.5 g/cc. Insulating paperboard properties are given in Table 1. TABLE 1 Insulating Paperboard Properties % of Mixture Overall Taber Sample Basis Wt. % Mixture Processed Crosslink Caliper Density, Bulk, ZDT Stiffness ΔT, No. Wt. g/m² D. Fir Total, % Fiber Crosslink Level, % (mm) g/cc cm³/g (kPa) (g-cm) ° C. 1 217.8 35 60 30 70 42 0.68 0.32 3.13 303.4 104.9 6.3 2 208.7 35 60 50 50 30 0.57 0.36 2.75 448.2 96.7 5.3 3 210.9 35 60 70 30 18 0.52 0.41 2.47 624.0 87.2 3.6 4 373.5 47.5 47.5 30 70 33.25 1.00 0.37 2.69 371.6 434.2 8.9 5 380.4 47.5 47.5 50 50 23.75 0.95 0.40 2.50 502.0 446.3 8.4 6 372.9 47.5 47.5 70 30 14.25 0.87 0.43 2.32 589.5 406.0 7.4 7 548 35 35 30 70 24.5 1.35 0.41 2.47 424.0 1062.2 12.2 8 552.2 35 35 50 50 17.5 1.29 0.43 2.34 466.1 974.1 10.9 9 539.9 35 35 70 30 10.5 1.13 0.48 2.1 678.5 968.7 9.8 10 216.4 35 60 30 70 42 0.78 0.28 3.63 179.3 115.7 8.8 11 217.2 35 60 50 50 30 0.73 0.30 3.38 207.5 109.9 8.0 12 216.4 35 60 70 30 18 0.71 0.30 3.3 255.1 113.0 7.3 13 389.2 47.5 47.5 30 70 33.25 1.13 0.34 2.91 206.9 332.3 11.3 14 392.6 47.5 47.5 50 50 23.75 1.09 0.36 2.77 281.3 413.7 10.6 15 390.9 47.5 47.5 70 30 14.25 1.06 0.037 2.72 359.9 444.6 9.3 16 559.2 60 35 30 70 24.5 1.4 0.40 5.50 337.9 859.5 12.5 17 565.9 60 35 50 50 17.5 1.4 0.41 2.47 353.7 938.4 11.7 18 565.5 60 35 70 30 10.5 1.34 0.42 2.36 428.9 822.2 11.4 Note: Samples 1-9, inclusive, contained CTMP fibers as the processed fibers in the mixture with crosslinked fibers and samples 10-18 contained TMP fibers as processed fibers in the mixture with crosslinked fibers. All samples contain 5% Lodgepole Pine, by total dry weight of fiber, refined to 50 CSF.

In another embodiment the paperboard exhibits a ΔT of at least 3.6° C. at a caliper of 0.5 mm. In another embodiment the paperboard exhibits a ΔT of at least 9.8° C. at a caliper of at least 1.15 mm. The relationship of ΔT (as defined below) to caliper is a linear one between the caliper of 0.5 mm and 1.15 mm and continues to be linear with a reduction in the caliper below 0.5 mm or an increase above 1.15 mm.

These temperature values are based on a linear regression equation of caliper versus ΔT using the values for caliper and ΔT for samples 1-9 in Table 1. Upper and lower confidence limits can be calculated for each point on the regression line from the data given in Table 1. The statistical parameters are given in Table 2. TABLE 2 Regression Statistics Multiple R .999 R Square .98 Lower Upper Coefficients 95.0%* 95.0%* Intercept −0.32 −1.55 0.9 X Variable 9.06 7.8 10.3 *Confidence Limits

Using the coefficients established in Table 2 above, the following relationships in Table 3 can be established for the ΔT at different caliper levels for samples 1-9, inclusive. TABLE 3 ΔT At Various Caliper Levels Based On Regression Line Caliper ΔT, ° C. LCL UCL 0.2 1.5 0 3.0 0.3 2.4 .8 4.0 0.4 3.3 1.6 5.0 0.5 4.2 2.4 6.1 0.6 5.1 3.1 7.1 0.7 6 3.9 8.1 0.8 6.9 4.7 9.2 0.9 7.8 5.5 10.2 1 8.7 6.3 11.2 1.1 9.6 7.0 12.3 1.2 10.5 7.8 13.3 1.25 11.4 8.6 14.3 LCL, Lower 95% Confidence Limit UCL, Upper 95% Confidence Limit

Tables 4 represents the regression statistics for samples 10-18 in which TMP was used in the mixture with crosslinked fiber and Table 5 represents the ΔT values at various caliper levels using the coefficients and confidence limits established in Table 4. TABLE 4 Regression Statistics Multiple R 0.95 R Square .91 Lower Upper Coefficients 95.0%* 95.0%* Intercept 3.48 1.54 5.41 X Variable 6.19 4.43 7.94 *Confidence Limits

TABLE 5 ΔT At Various Caliper Levels Based On Regression Line Caliper ΔT, ° C. LCL UCL 0.2 4.7 2.4 7 0.3 5.3 2.9 7.8 0.4 5.9 3.3 8.6 0.5 6.6 3.8 9.4 0.6 7.2 4.2 10.2 0.7 7.8 4.6 11 0.8 8.4 5.1 11.8 0.9 9 5.5 12.5 1 9.7 6 13.3 1.1 10.3 6.4 14.1 1.2 10.9 6.9 14.9 1.25 11.5 7.3 15.7 LCL, Lower 95% Confidence Limit UCL, Upper 95% Confidence Limit

The paperboard of the application can be a single-ply product. When a single-ply product is employed, the low density characteristics of the paperboard of the present application allows the manufacture of a thicker paperboard at a reasonable basis weight. Using a mixture of the crosslinked cellosic fibers and processed cellulosic fibers of the present application, an insulating paperboard having the same basis weight as a normal paperboard can be made. This effectively allows the manufacture of insulating paperboard on existing paperboard machines with minor modifications and minor losses in productivity. Moreover, a one-ply paperboard has the advantage that the whole structure is at a low density. Alternatively, the paperboard of the application can be multi-ply product, and include two, three, or more plies. Paperboard that includes more than a single-ply can be made by combining the plies either before or after drying. Multi-ply paperboard can be made by using multiple headboxes arranged sequentially in a wet-forming process, or by a baffled headbox having the capacity of receiving and then laying multiple pulp furnishes. The individual plies of a multi-ply product can be the same or different.

The paperboard of the present application can be formed using conventional papermaking machines including, for example, Rotoformer, Fourdrinier, inclined wire Delta former, and twin-wire forming machines.

In one embodiment when a single-ply paperboard is used in accordance with the present application, it is homogeneous in composition. The single ply, however, may be stratified with respect to composition and have one stratum enriched with crosslinked cellulosic fibers and processed cellulosic fibers and another stratum enriched with cellulosic fibers (chemical pulp fibers) to provide a smooth, denser, less porous surface.

It is most economical to produce a composition paperboard that is homogeneous where the crosslinked cellulosic fibers and processed cellulosic fibers are uniformly mixed with the cellulosic fibers. In one embodiment the crosslinked cellulosic fibers and processed cellulosic fibers are present in the insulating ply or layer in an amount from about 35% to about 60% of the total dry weight of the cellulosic fiber. In another embodiment they are present in an amount of from 45% to about 55%. of the total dry weight of the cellulosic fiber. In a two-ply structure, for example, the first ply may contain 100% cellulosic fibers while the second ply may contain from 25% to 70% crosslinked cellulosic fibers and processed cellulosic fibers. In another embodiment the second ply may contain from 35% to 60% crosslinked cellulosic fibers and processed cellulosic fibers. In one embodiment, in a three-ply layer, the bottom and top layers may comprise 100% of cellulosic fibers while the middle layer may contain from about 25% to about 70% of crosslinked cellulosic fibers and processed cellulosic fibers. In another embodiment, in a three ply layer, the middle layer may contain from about 35% to about 60% of crosslinked celluosic fibers and processed cellulosic fibers.

The paperboard of the present application has a broad set of strength properties. For example, in one embodiment the Taber stiffness ranges from about 90 g-cm to about 1000 g-cm. In another embodiment the Taber stiffness ranges from about 150 to about 500 g-cm and in yet another embodiment the Taber stiffness ranges from about 200 to about 400 g-cm. Taber stiffness was determined by TAPPI T-489.

In converting operations of a conventional board to the cup, a minimum Z-direction tensile (ZDT) is necessary for proper rim or top curl formation so that delamination does not occur during this process. In one embodiment ZDT ranges from about 180 kPa to 450 kPa, in another embodiment the ZDT ranges from about 300 kPa to about 400 kPa. ZDT was determined by TAPPI T-541.

Sheet bulk was determined by TAPPI T-411.

The paperboard of the present application can be utilized to make a variety of structures, particularly containers, in which it is desired to have insulating characteristics. Referring to FIG. 2, one of the most common of these containers is the ubiquitous hot cup utilized for hot beverages such as coffee, tea, and the like. Other food service items that could benefit from improved insulating properties such as noodle cups, and press-molded plates and bowls can also incorporate the paperboard of the present application. Also, carry-out containers conventionally produced of paperboard or of foam material can also employ the paperboard of the present application. As shown in FIGS. 2 and 3, a hot cup type container produced in accordance with the present application may comprise one or more plies 22 and 24, one of which, in this instance, 24, contains crosslinked cellulosic fibers and processed cellulosic fibers. In this embodiment the crosslinked cellulosic fibers and the processed cellulosic fibers are in the interior ply 24. A liquid impervious backing 26 is preferably laminated to the interior ply. The backing may comprise, for example, a variety of thermoplastic materials, such as polyethylene. It is preferred that the paperboard used in the bottom of the cup contain no crosslinked cellulosic fibers or processed cellulosic fibers.

ΔT Test Procedure

Paperboard thermal performance is determined in a test unit that models the heat transfer through a paper cup. A box of plexiglass measuring 10×10×10 cm interior dimensions has a sample opening of 8.2 cm by 8.2 cm in one side. A gasket of surgical tubing is attached to the box around the perimeter of the 8.2 cm×8.2 cm opening. A 10 cm×10 cm sample of paperboard is laminated on one surface with 10 cm wide 3 M Tartan 3765 packaging tape. Alternatively, polyethylene may be extruded onto the surface of the board. The paperboard sample is mounted onto the apparatus covering the sample opening with the sealed face toward the interior. A separate piece of plexiglass (with the same outside dimensions as the box and a hole 8.2 cm×8.2 cm cut out) is clamped over the paperboard sample to hold it firmly against the box. The box is filled with hot water at a temperature of 96.1° C. (205° F.) through a small opening in the top of the box so that the water is in full contact with the sample. A small stir bar is inserted into the box and the assembly is then placed on a stir plate to permit stirring during the measurement phase. A K type thermocouple is inserted into the hot water through the small opening in the box top and an infra-red thermometer IRCON Inc. Modline Series 3400 Radiation Thermometer, set to measure at 0.96 emissivity is aimed at the outside center of the paperboard sample at a 29.7 cm distance and the IR radiation measured. A data logger, (HP34970A Data Acquisition/Switch Unit capturing the mVdc response from the radiation thermometer adjusted by a gain of 30.0 and an offset of 100 and the mVdc response from the thermocouple but does not adjust it) records the temperature of both the inside water (from the thermocouple inserted into the water) and the outside surface of the sample (from the infrared radiation thermometer gun) from which the temperature drop (ΔT) can be calculated. When the water temperature reaches 85° C. (185° F.), the data capture is halted. The difference between the inside water temperature and the outside paperboard temperature is calculated for each data point captured by the data logger. A linear regression analysis is performed on the data for ΔT (inside water temperature minus outside wall temperature) versus inside water temperature and, from the regression, the ΔT at 87.8° C. (190° F.) is determined. The linear regression analysis is run from the point of maximum outside wall temperature to a point on the curve that corresponds to an internal water temperature of 85° C. (185° F.). ΔT is the difference in temperature between the water temperature of 87.8° C. (190° F.) and the corresponding outside wall temperature of the paperboard on the test unit.

Insulating paperboard with varying compositions of crosslinked fiber and processed cellulosic fibers, basis weight and other properties, shown in Table 1, were made by methods similar to those represented in Examples 1 and 2 by substituting the appropriate amount of fiber and additives.

EXAMPLE 1

This method is representative of making a 350 gsm board in which 60% of the total dry weight of the fiber mixture is made up of a 50/50 blend of crosslinked fiber and Bowater TMP fiber. In all cases, dry weight of fiber means the fibers were dried at 105° C. for one hour. Other paperboards, shown in Table 1, of various basis weights and various levels of crosslinked fibers and processed fiber levels can be made with adjustment to the appropriate amounts and weights of fiber and other additives. In all samples shown in Table 1 the bleached Douglas Fir component was refined to 510 CSF; Lodgepole Pine (refined to 50 CSF) was added to all samples at a level of 5% of total dry fiber weight. All samples had 10% by weight PVOH added on the total fiber dry weight. Other additive levels are given in this Example and in Example 2.

TMP (from Bowater), 19.43 g fiber (43.2% consistency), 57.7 g Douglas Fir refined to 510 CSF (29.1% consistency), 9.03 crosslinked fiber (93.0% consistency), 70.7 g Lodgepole Pine refined to 50 CSF (2.5% consistency), and 3.54 g polyvinylalcohol (Celvol 165SF PVOH, available from Celanese, Dallas Tex.), 100% solids, were disintegrated for 5 minutes in a British Disintegrator. The mixture was diluted to 4 L with deionized water and adjusted to a pH of 7.2-7.4 using NaHCO₃. The equivalent of 1 g/kg (2 lb/T) Kymene and 0.13 g/kg (0.26 lb/T) of Perform-PC8138 (both available from Hercules, Wilmington, Del.) were added from 1% solutions each, and mixed for 2 minutes. AKD (alkyl ketene dimer available from Hercules, Inc., Wilmington, Del.) at 2 g/kg (4 lb/T) and 4.25 g/kg (8.5 lb/Ton) starch (Sta-Lok 300, available from Tate-Lyle, Decatur Ill.) were each added and the mixture stirred for two minutes. A 31.75×31.75 cm forming wire (155 mesh) was placed in the bottom of a Noble & Wood 12.5″ by 12.5″ handsheet mold, the slurry poured into the sheet mold, diluted to 35 liters with deionized water and mixed with a plunger. The slurry was then drained, dewatered by using blotters with even hand pressing until the sheet reached a consistency of approximately 20%. The sheet was removed from the screen and blotted further to approximately 30% solids. Blotters were placed on each side of the sample, the sample placed between damp felts and then passed through a press at 137.8 kPa (20 psi) to further dewater the sample. The solids content at this point was approximately 40%. The resulting sheet was placed on a drum dryer, (surface temperature of 121° C.), between two dry blotters and allowed to dry for 10 minutes. The sample was then inverted and allowed to dry an additional 10 minutes. The sample was conditioned in a 50% Relative Humidity room for a minimum of 4 hours prior to testing.

EXAMPLE 2

This method is representative of making a 200 gsm board in which 60% of the total dry weight of the fiber mixture is made up of a blend of 42% crosslinked fiber and 18% Bowater TMP fiber. Other paperboards, shown in Table 1, of various basis weights and processed fiber levels can be made with adjustment to the appropriate amounts and weights of fiber and other additives. In all samples shown in Table 1 the bleached Douglas Fir component was refined to 510 CSF; Lodgepole Pine, refined to 50 CSF, was added to all samples at a level of 5% by weight of total dry fiber. All samples had 10% by weight PVOH added on the total fiber dry weight.

CTMP, 8.35 g fiber (43.5% consistency), 24.3 g Douglas Fir refined to 510 CSF (29.1% consistency), 9.1 g crosslinked fiber (93.0% consistency), 40.5 g Lodgepole Pine refined to 50 CSF (2.5% consistency), and 2.02 g polyvinylalcohol (Celvol 165SF PVOH, available from Celanese, Dallas Tex.), 100% solids, were disintegrated for 5 minutes in a British Disintegrator. The mixture was diluted to 4 L with deionized water and adjusted to a pH of 7.2-7.4 using NaHCO₃. The equivalent of 1 g/kg (2 lb/T) Kymene and 0.13 g/kg (0.26 lb/T) of Perform-PC8138 (both available from Hercules, Wilmington, Del.) were added from 1% solutions each, and mixed for 2 minutes. AKD (alkyl ketene dimer available from Hercules, Inc., Wilmington, Del.) at 2 g/kg (4 lb/T) and 4.25 g/kg (8.5 lb/Ton) starch (Sta-Lok 300, available from Tate-Lyle, Decatur Ill.) were each added and the mixture stirred for two minutes. A 31.75×31.75 cm forming wire (155 mesh) was placed in the bottom of a Noble & Wood 12.5″ by 12.5″ handsheet mold, the slurry poured into the sheet mold, diluted to 35 liters with deionized water and mixed with a plunger. The slurry was then drained, dewatered by using blotters with even hand pressing until the sheet reached a consistency of approximately 20%. The sheet was removed from the screen and blotted further to approximately 30% solids. Blotters were placed on each side of the sample, the sample placed between damp felts and then passed through a press at 137.8 kPa (20 psi) to further dewater the sample. The solids content at this point was approximately 40%. The resulting sheet was placed on a drum dryer, (surface temperature of 121° C.), between two dry blotters and allowed to dry for 10 minutes. The sample was then inverted and allowed to dry an additional 10 minutes. The sample was conditioned in a 50% Relative Humidity room for a minimum of 4 hours prior to testing.

The foregoing application has been described in conjunction with a preferred embodiment and various alterations and variations thereof. One of ordinary skill will be able to substitute equivalents in the disclosed application without departing from the broad concepts imparted herein. It is therefore intended that the present application be limited only by the definition contained in the appended claims. 

1. An insulating paperboard comprising: at least one layer comprising a mixture of crosslinked fibers and processed cellulosic fibers, said crosslinked cellulosic fibers and said processed fibers being present in an amount in said mixture from 35 to 60 percent of total dry weight of fibers of said at least one layer, and wherein said paperboard being sufficiently insulating to provide a ΔT across said paperboard of at least 3.6° C. at a caliper of 0.5 mm, said paperboard having a density of less than 0.5 g/cc, and a basis weight of from 200 gsm to 500 gsm, and said caliper of said paperboard being greater than or equal to 0.5 mm.
 2. The crosslinked fibers of claim 1 wherein said crosslinked fibers are present in an amount of 30 to 70 percent of the total dry weight of the mixture and the processed fibers are present in an amount of 70 to 30 percent of the total dry weight of the mixture.
 3. The processed fibers of claim 2 wherein the processed fibers are selected from the group consisting of chemically processed fibers, mercerized fibers, mechanically processed fibers, chemimechanically processed fibers, jet dried fibers, flash dried fibers and mixtures thereof.
 4. The fibers of claim 3 wherein the processed fibers are CTMP fibers.
 5. The fibers of claim 3 wherein the processed fibers are TMP fibers.
 6. The crosslinked fibers of claim 1 wherein the fibers are selected from the group consisting of citric acid crosslinked fibers, polyacrylic acid crosslinked fibers and polymaleic acid crosslinked fibers.
 7. The crosslinked fibers of claim 6 wherein the crosslinked fibers are polyacrylic acid crosslinked fibers.
 8. The crosslinked fibers of claim 6 wherein the crosslinked fibers are citric acid crosslinked fibers.
 9. The insulating paperboard of claim 1, wherein said paperboard has a basis weight of from 250 gsm to 450 gsm.
 10. The insulating paperboard of claim 1, wherein said paperboard has a ΔT of at least 3.6° C. at a caliper of 0.5 mm and a ΔT of 9.8° C. at a caliper of 1.15 mm, said ΔT being a substantially linear progression relative to caliper in the range from below a ΔT of 3.6° C. to above a ΔT of 9.8° C.
 11. The insulating paperboard of claim 10, wherein said linear progression extends from a ΔT of 3.6° C. to a ΔT of 9.8° C.
 12. The paperboard of claim 1 wherein the Taber stiffness is at least 150 g-cm.
 13. The paperboard of claim 1 wherein the ZDT is at least 180 kPa.
 14. The insulating paperboard of claim 1, wherein said paperboard is at least a two-ply board, said at least one ply containing said crosslinked cellulosic fibers and processed cellulosic fibers. 